Distributed and secured method and system for protecting and distributing audio-visual flows

ABSTRACT

A process for secure distribution of digital audiovisual streams according to a standard, normalized or proprietary format, on which streams a separation of the stream into two parts is made prior to transmission to equipment of an addressee to generate a modified main stream having a format of the original stream and complementary information of any format including digital information suitable to permit reconstruction of the original stream, wherein a transmission is made, by separate paths during distribution of the modified main stream from a distribution server and of the complementary information to the equipment of the addressee from a secure central server passing via at least one local server connecting the equipment of the addressee to the local server via at least one access point.

RELATED APPLICATION

This is a continuation of International Application No.PCT/FR2004/050381, with an international filing date of Aug. 11, 2004(WO 2005/018232 A2, published Feb. 24, 2005), which is based on FrenchPatent Application No. 03/50423, filed Aug. 11, 2003.

FIELD OF THE INVENTION

This invention relates to the area of the distribution of digitalaudiovisual sequences.

BACKGROUND

It is currently possible to transmit audiovisual programs in digitalform via broadcasting networks of the microwave (herzian), cable,satellite type, etc. or via telecommunication networks of the DSL type(Digital Subscriber Line) or BLA type of (Local Radio Loop) or via DABnetworks (Digital Audio Broadcasting) as well as via any wirelesstelecommunication network of the GSM, GPRS, UMTS, Bluetooth, WiFi types,etc. These works are frequently encrypted or scrambled by various wellknown means to avoid pirating.

As concerns the distributed systems based on the client-server principlecharacterized by “caching,” the prior art also contains two main typesof systems that are classified according to the content treated by this“caching.” The term “caching” denotes the possibility of being able totemporarily hold a copy of contents or of data (permanently stored in acentral server) at a point or at different points of the network (e.g.,local servers) to serve the requests of clients the closest to thesepoints, thus reducing the overload on the server from contents andconsequently optimizing the transmission rate used at the access points.

The first type processes data whose distribution has no timerestrictions (systems for the distribution of files by “caching”) andthe second type concerns the processing of multimedia (audio/video)data.

The systems for conventional distributed files such as Sun NFS, ApolloDomain, Andrew, IBM AIX, DS, AT&T RFS perform the “caching” of fileslocally, do not have the possibility of making the “caching” of files inproximate or remote nodes and cannot allocate local servers for applyingthe “caching” to files. Moreover, the conventional distributed systemscharacterized by “caching” have a granularity of the size of a file andconsequently the possibilities of having a scalability of thedistribution of the contents via the network is greatly reduced.

On the other hand, other distributed multimedia systems characterized by“caching” such as the “Berkeley Distributed VOD,” for example, do notobtain a “caching” that is completely secure and personalized for eachuser and also have limited capacities of scalability while frequentlybeing penalized by the limited bandwidth of the networks.

SUMMARY OF THE INVENTION

This invention is related to a process for secure distribution ofdigital audiovisual streams according to a standard, normalized orproprietary format, on which streams a separation of the stream into twoparts is made prior to transmission to equipment of an addressee togenerate a modified main stream having a format of the original streamand complementary information of any format including digitalinformation suitable to permit reconstruction of the original stream,wherein a transmission is made, by separate paths during distribution ofthe modified main stream from a distribution server and of thecomplementary information to the equipment of the addressee from asecure central server passing via at least one local server connectingthe equipment of the addressee to the local server via at least oneaccess point.

This invention also relates to a system that securely distributesaudiovisual streams according to the process, including a device forseparating the original video stream into a modified main stream andinto complementary information, at least one multimedia servercontaining protected audiovisual streams, at least one secure centralserver from which the complementary information is distributed, at leastone telecommunication network, at least one local server communicatingwith at least one access point for connection to the addressed equipmentand a device on the addressed equipment for reconstructing the originalaudiovisual stream as a function of the modified main stream and thecomplementary information.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a client-server system in accordance withselected aspects of the invention.

FIG. 2 is a schematic representation of the structure of a unit of acomplementary information stream in accordance with selected aspects ofthe invention.

FIG. 3 is a schematic representation of the composition of an accessticket used in the distribution system in accordance with aspects of theinvention.

FIG. 4 is a schematic representation of segments in a stream ofcomplementary information in accordance with aspects of the invention.

DETAILED DESCRIPTION

This invention provides a process and system that permit 1) the visualand/or auditory protecting of an audiovisual sequence stemming from adigital standard, a digital norm or a proprietary standard, thedistributing of it via a telecommunication network distributed in asecure manner and 2) reconstitution of the original content from adigital audiovisual stream on a recomposition module of the addressee'sequipment.

The invention relates more particularly to a device capable oftransmitting in a secure manner and via a distributed network a set ofhigh-quality audiovisual streams to a viewing screen and/or to an audiooutput belonging to a terminal or display device such as a televisionscreen, a computer or even a mobile telephone, a mobile iPDA terminal ora PDA (Personal Digital Assistant) or the like while preserving theaudiovisual quality, but avoiding fraudulent use such as the possibilityof making pirated copies of the broadcast contents.

The invention relates to a process and a client-server system thatprotects the audiovisual contents by separating them into two parts, thesecond part of which is indispensable for reconstitution of the originalstream, the latter being restored as a function of recombining the firstpart with the second part.

For example, the process may separate the audiovisual stream into twoparts in such a manner that the first part, called “modified mainstream,” contains the quasi totality of the initial information, forexample, more than 95%, and a second part, called “complementaryinformation,” containing targeted elements of the initial informationand which is of a very small size compared to the first part.

In distinction to the prior art this invention provides a systemcharacterized by “caching” in the sense in that it processes data inreal time, but with the particularity that the processing is performedon connected elements of segments that are independent entities from theprocessing point of view and the “caching” point of view, which segmentscarry data for the reconstruction of the complete audiovisualinformation and are personalized for each user and sent to the equipmentof addressees in real time via a low bandwidth network of a local serverplaying the role of controller for securing the content.

The protection applied to the content distributed by the securedistributed system is advantageously based on the principle of thesuppression, removal or deletion and replacement of certain informationpresent in the original encoded audiovisual signal by any method, e.g.:substitution, modification, permutation or shifting of the information.This protection is also based on a knowledge of the structure of thestream. The solution is extracted and permanently retained in a secureserver connected to the broadcasting and transmitted network in thecomplementary information a part of the audiovisual program recorded atthe user's or directly broadcasted, which part is of primary importancefor reconstituting the audiovisual program on a screen or on an audiooutput of a terminal, but has a very low volume relative to the totalvolume of the digital audiovisual program recorded at the user's orreceived in real time. The missing part may be transmitted via thesecure, distributed broadcasting or transmitting network at the momentof viewing or listening to the audiovisual program. The data removed issubstituted by random or calculated data called decoys.

The fact of having removed and substituted with decoys a part of theoriginal data of the traditional audiovisual stream during generation ofthe modified main stream does not permit restitution of the originalstream only from the data of the modified main stream. The modified mainstream is totally compatible with the format of the original stream andcan therefore be copied and read by a reader, but it is completelyincoherent from the viewpoint of human visual and auditory perception.

Once the digital stream is separated into two parts, the largest part ofthe audiovisual stream, the modified main stream, is then transmittedvia a classic broadcasting network whereas the lacking part, namely thecomplementary information, is sent on demand via a narrow bandtelecommunication network such as the classic telephone or cellularnetworks of the GSM, GPRS or UMTS or by using a small part of a networkof the DSL or BLR type, or by using a subset of the broadband shared ona cable network, or also via a physical support such as a memory card orany other support. However, the two networks can be combined whileretaining the two separate transmission paths. The audiovisual stream isreconstituted on the addressee's equipment by a synthesizing module fromthe modified main stream and the complementary information sent piece bypiece during use of the audiovisual stream.

Selected aspects of the subject matter of this invention relates to thesecure transmission, after identification and localization of the user,of the complementary information via a distributed network in such amanner as to prevent it from being able to be copied or totally fallinginto the possession of the user or of any bad-intentioned person.

Aspects of the invention also concern a process for the securedistribution of digital audio streams according to a standard,normalized or proprietary format, on which streams a separation of thestream into two parts is made prior to the transmission to the equipmentof the addressee to generate a modified main stream having the format ofthe original stream and complementary information of any formatcomprising the digital information suitable for permitting thereconstruction of the original stream, characterized in that atransmission is made, by separate paths during the distribution phase,of the modified main stream from a distribution server and thecomplementary information to the equipment of the addressee from asecure central server passing via at least one local server connectingthe equipment of the addressee to the local server via at least oneaccess point.

The secure central server is preferably administered by a trusted thirdparty.

The process can optionally have the following additionalcharacteristics:

the secure central server performs segmentation of the complementaryinformation with each segment corresponding to a subjectively coherententire audiovisual element in units of streams of complementaryinformation with a variable size,

the units of streams of complementary information are organized inseveral layers corresponding to the layers of scalability of theoriginal audiovisual stream,

prior to sending to a local server, the secure central server carriesout encryption of the units of the streams of complementary informationwith a first encryption key,

before sending the complementary information from the local server tothe addressed equipment, the complementary information is decrypted withthe first encryption key and re-encrypted with a second encryption key,

encryption of the complementary information with the second key isadaptive as a function of the throughput (transmission rate) capacitiesof the addressed equipment,

the second encryption key is constructed during the authentication stagebetween the addressed equipment and a local server,

the local server generating a session key with the client becomes a hostserver of the session during the entire lifetime of the session key,

the second encryption key is valid solely during the time of acontinuous session between the addressed equipment and at least onelocal server,

an access ticket is generated per session, containing informationconcerning the second session key, valid solely during the session andexpiring at the end of the validity of the access ticket,

distribution of the complementary information onto a distributed networkhas the property of scalability in the transmission rate,

distribution of the complementary information is carried out by applyingmechanisms for regulating the quantity of information distributed intothe distributed network, taking account of the capacities and therestrictions of the distributed network in terms of storage and accesstime.

Other aspects of the invention also relate to a system for the securedistribution of audiovisual streams for carrying out the processcomprising a device for the separation of the original video stream intoa modified main stream and complementary information, at least onemultimedia server containing the protected audiovisual streams, at leastone secure central server from which the complementary information isdistributed, at least one telecommunication network, at least one localserver communicating with at least one access point for the connectionto the addressed equipment and a device on the addressed equipment forthe reconstruction of the original audiovisual stream as a function ofthis modified main stream and of the complementary information.

Selected aspects of the invention will be better understood with the aidof exemplary embodiments and stages discussed in detail in thefollowing.

A preferred, but non-limiting exemplary embodiment of the process thatmeets the criteria of security and reliability is illustrated by theclient-server system shown in FIG. 1.

The audiovisual stream in digital form 1 is separated into two parts byanalysis and scrambling module 2. Modified main stream 14 is stored inmultimedia server 13 and sent in real time to the client via broadbandnetwork 12 or stored in advance on the backup device of terminal 11 ofthe user. Complementary information 3 is sent to storage andsegmentation module 41 of secure central server 4.

Once the complementary information is sent solely on demand, itsdistribution in real time and its personalization for each user isrealized by virtue of the property of scalability in the transmissionrate on the transport networks. The notion of “scalability in thetransmission rate” is defined as the capacity of a network to manage,modify, distribute and adapt the transmission rate of the streamstransiting as a function of the available or negotiated broadband and asa function of the network congestions. In order to respond to therequirements of sending in real time and by virtue of the lowtransmission rate of the complementary information for the transmission,the process comprises a stage for the segmentation of the complementaryinformation in module 41, that generates data segments of variable size,each segment corresponding to an entire, subjectively coherentaudiovisual element such as an image or a frame, a group of images orGOP (Group of Pictures) in an MPEG-2 stream, for example. Thesegmentation may be carried out in a single stage after generation ofthe complementary information 3 and produces a series of segmentsdesignated as “streams of complementary information” that remain storedin storage and segmentation module 41. The stream of complementaryinformation may be generated in real time.

The segmentation stage of the complementary information is followed by astage of encapsulation in blocks of data and a stage of encryption inmodule 42 where it remains available at the request of local servers 6.The stream of complementary information is continuously sent to terminal1 of the user in the form of blocks, a block containing a segment towhich access or “header” information was added comprising data relativeto the mobility of the user (position, rights, network access points,for example) and data relative to the encryption keys of the stream ofcomplementary information. A block is the fundamental communication unitand is also called UFIC (unit of a complementary information stream).

When the user 11 wishes, e.g., to view a sequence, the user connects viaequipment to the module for managing access points 9 that redirects therequests to a local server 6 that addresses for its part the request tocentral server 4 in the case of a first request relative to this stream.

Central server 4 requires an authentication on the part of local servers6 when it receives the request to decide about sending streams ofcomplementary information requested, which is solely by way ofaudiovisual sequence. The authentication dialog is established with thecentral server and, after local server 6 has been recognized by centralserver 4, the stream segmented in module 41, sent via link 43 to module42 and encrypted in module 42 by a first key that is unique by title andby local server, is transported via link 5. Each stream of complementaryinformation is sent to the local server in encrypted form with the firstkey that is unique regarding the stream for each server.

FIG. 2 presents the structure of a unit of a complementary informationstream. A universal identification unit (IUU) is found at the start ofeach unit, coded, e.g., with 8 bytes, that is considered as a universaladdress (e.g., a URL (“Universal Resource Locator”) in the World WideWeb system, by means of which the server can localize the UFICrequested. The next field is reserved for the access rights that definethe access and the decryption of the elements of the complementaryinformation stream necessary for the recomposition of the audiovisualstream. This field is updated by encapsulation and encryption module 42when central server 4 receives a request from a local server. This fieldis advantageously also modified by the local server as a function of theuser rights.

Another field contained in the UFIC is the “data length” field, thatcontains the size, e.g., in bytes, of the encrypted UFIC followed by the“data” field containing the values of the encrypted UFIC. The structureends with an indicator designating the address of the next UFIC requiredfor this stream with the aid of a differential address.

The data contained in the “marker” field is relative to thecharacteristic of scalability of the transmission rate of the system,that is, to the rate of transmission/reception and the processingcapacity for decrypting the UFIC's.

For example, since the transmission rates used in mobile terminals ofthe portable telephone type, PDA (“Personal Digital Assistant”) are low,an adaptive method for encrypting UFIC's as a function of the bandwidthavailable or negotiated for each user of the mobile network is used.

Each UFIC contains in the “marker” field an indication coded on a bitabout its own status: encrypted or non-encrypted. The local serverbegins with a negotiation phase about the capacities of the user'sterminal in terms of bandwidth and about the price that the latter isready to pay, and decides whether the encryption mode is complete orpartial, which is indicated on the access ticket. The partial encryptionis used between the local servers and the clients whereas a completeencryption is always applied between the central server and the localservers.

Local server 6 contains a list of audiovisual programs available incentral server 4. As the storage capacities in the local servers arelimited, e.g., in the case in which several clients are simultaneouslyconnected, these servers make use of mechanisms for replacing data inthe case that their storage capacities are exceeded or overloaded.

For example, one method of managing the storage is the method of theleast used recent elements or LRU (“Least Recently Used”). Anothermethod is the BE-LRU (Back-End LRU). The server manages the place forthe new entering data by replacing streams that were not recentlyaddressed by beginning with the UFIC's of the end of these streams, thusassuring a granularity (precision) of replacement equal to one UFIC.Thus, this mechanism allows minimization of the frequency of effectivereplacements of different streams. Also, an uploading in advance(“pre-fetching”) of the complementary information stream of centralserver 4 to local servers 6 is applied for the efficient managing of thecentral server. In this manner, a too great number of requests from thelocal servers is avoided. The method used is the inverse of that of thereplacement, that is to say that the uploading is carried out bybeginning with the recently most addressed streams and a part of thebeginning (corresponding, e.g., to the first minutes of the audiovisualsequence) of these streams is transferred to the local servers.

A dialog is then established between local server 6 and viewing terminal11 based on a method of authentication to generate an access ticket thatcontains information concerning a second encryption key. The second keyis valid only during the session and expires at the end of the validityof the access ticket, which access ticket contains information about thehost server, about the lifetime of the key, but never the key itself.The second key is called the “session key,” with which the local serverre-encrypts the UFIC's just before sending to terminal 11 after havingdecrypted them with the first key. Such a ticket system permits theclient to have a key valid for a limited time period for the recovery ofa complementary information stream during the “lifetime” (“Time To Live”(TTL)) of the session key. The management of the access tickets iscarried out by local server 6 and the access ticket is valid even if theuser changes the network access point and as a consequence changes thelocal server, by virtue of the system for distributing and updating theticket in a geographic zone 7.

FIG. 3 shows the composition of the access ticket used in thedistributed system. The first four bytes correspond to the identity ofthe geographic zone 7 in which the local server is situated and arefollowed by four bytes representing the identity of the server that is agenerator of the session key corresponding to this ticket. The twofollowing bytes contain the information on the lifetime of theassociated session key. The last couple of bites is reserved forinformation linked to the service profile between the addressed terminaland the local server, e.g., the decision to apply a type of partialencryption corresponding to the bandwidth allocated to the terminal.

The distribution of tickets and their updating valid in a “geographiczone” 7 is carried out as a function of the communication protocolbetween the servers of the same zone (6 i and 6 j). The client 11 sendsa request to retrieve the complementary information to server 6 i vialink 10 and access point 9 i and the access ticket is generated. Server6 i is then called the host server (“key host server”). Server 6 i sendsthe UFIC's via link 8 i, network access point 9 i and link 10 i. Iftraveling client 11 i moves and changes the access point 9 j in the samegeographic zone 7, the UFIC requests are then addressed to local server6 j. Server 6 j determines by the access ticket that it is not the hostserver of the client. At this moment, server 6 j calls all the localservers of the same geographic zone 7 until host server 6 i has beenidentified and it requests for its part validation of the current accessticket in the case that local server 6 j has not validated it inadvance. If the validation of the access ticket is confirmed by server 6i, server 6 j verifies that it has the UFIC's corresponding to the sameinitial audiovisual stream, recovers the information about the sessionkey of the client 10 i, generates the same key and sends it theencrypted UFIC's either up to the end of the validation of the accessticket or while the client remains connected at this access point 9 j.In the case that validity of the access ticket expires, server 6 jremakes an authentication and thus becomes the host server of client 10i. Another possibility for distributing and updating access tickets isbroadcasting of information concerning the duration of the validity oftickets generated by host server 6 i to all local servers 6 j comprisedin the same geographic zone 7.

As described above, authentication is carried out in the local server togenerate a different session key for each client, referenced in theaccess ticket with limited validity in time. Authentication is made uponthe request of the client, who is informed about the duration of thevalidity of the session key. The UFIC's sent via elements 8, 9 and 10are decrypted at the client's terminal 11 with the session key by, meansof, e.g., a smart card 15 integrated in audiovisual terminal 11 of theclient. This smart card also establishes authentication at the beginningof the session. Furthermore, the session key is generated in the localserver and at the client's by means of a “challenge.” The “challenge”represents generation and exchange of information fabricated randomlyfrom predefined rules known by both parties. The session key generatedis retained in memory in the local server and in the smart card of theclient equipment without ever being exchanged via the network.

The UFIC's decrypted by the smart card are used by the recompositionmodule integrated in the terminal of the user, that generates anaudiovisual stream strictly identical to original stream 1 from themodified main stream and the complementary information is stored by theUFIC's.

The stream of complementary information may be in the form of a group ofsubstreams of complementary information corresponding to a single,continuous auto visual stream (coming from an interactive televisionchannel or from a satellite channel, for example). The encoding of typeH264 advantageously furnishes the possibility of generating a set ofstreams corresponding to a single audiovisual sequence, with each sethaving a different number of frames per second. A given set is sent as afunction of the available transmission rate capacity of the network. If,e.g., the network frees up resources and there is thus the possibilityof sending a greater transmission rate (throughput), the setcorresponding to a greater number of images (pictures) per second isthen sent. The transition without discontinuity between the sets isassured by transition frames of the SI and SP type (“Switching I slice”and “Switching P slice”). In this exemplary aspect, each of these setscorresponds to a substream of complementary information.

Each of the substreams may correspond to a layer of scalability of thecontinuous audiovisual stream. The notion of “scalability” characterizesan encoder capable of encoding or a decoder capable of decoding anordered set of binary streams in such a manner as to produce orreconstitute a multilayered sequence.

The stream of complementary information may be unique and contains theunits corresponding to all the layers of scalability of the audiovisualstream. FIG. 4 shows an example. The segments containing thecomplementary information corresponding to the different layers ofscalability are arranged successively and a coding extension (1 bit)indicating the scalability presence is added followed by indications(coded on 2 bytes) for placing access points relative to each layer ofscalability.

An example for the streams of the MPEG-2 type characterized by theproperty of temporal scalability on two layers (base layer and anenhancement layer) is the insertion into the stream of complementaryinformation of access points relative to the part corresponding to thebase layer and to the part corresponding to the enhancement layer bysuccessive GOP.

Another example for streams of the MPEG-4 type characterized by theproperty of temporal scalability on two layers (base layer andenhancement layer) is the insertion into the stream of complementaryinformation of access points relative to the part corresponding to thebase layer and to the part corresponding to the enhancement layer bysuccessive GOV's (Group Of Video).

The substreams of complementary information corresponding to the baselayer and the enhancement layer are advantageously sent to the addresseeas a function of rights.

The substreams of complementary information corresponding to the baselayer and the enhancement layer are advantageously sent to the addresseeas a function of the network resources in terms of the allocatedtransmission rate.

The substreams of complementary information corresponding to the baselayer and the enhancement layer are advantageously sent to the addresseeas a function of the audiovisual quality required by the addressee.

The substreams of complementary information corresponding to the baselayer and the enhancement layer are advantageously sent to the addresseeas a function of the quality of service negotiated by the network.

1.-13. (canceled)
 14. An apparatus, comprising: a storage andsegmentation module configured to receive complementary informationextracted from an audiovisual stream and including digital informationsuitable for permitting reconstruction of the audiovisual stream inconjunction with a modified stream based on the audiovisual stream afterthe complementary information is extracted, the storage and segmentationmodule configured to store the complementary information and to segmentthe complementary information into respective segments corresponding toaudiovisual elements in units of streams of complementary informationwith a variable size; and an encryption module configured to encrypt thecomplementary information prior to transmitting the complementaryinformation to a local server for transmission to equipment of anaddressee, wherein the encryption module is configured to encrypt thecomplementary information using an encryption key determined by a titleof the original stream corresponding to the complementary informationand determined by the local server to which the complementaryinformation is to be transmitted.
 15. The apparatus according to claim14, wherein units of streams of complementary information are organizedin several layers corresponding to layers of scalability of theaudiovisual stream.
 16. The apparatus according to claim 14, whereinrespective segments correspond to a subjectively coherently entireaudiovisual element in units of streams of complementary informationwith a variable size.
 17. The apparatus according to claim 14, whereinthe apparatus is configured to transmit the complementary informationinto a distributed network with scalability in a rate at which thecomplementary information is transmitted into the distributed network.18. The apparatus according to claim 17, wherein a rate at which thecomplementary information is transmitted into the distributed network isdetermined based on at least one characteristic of the distributednetwork selected from the group consisting of: available bandwidth;negotiated bandwidth; and network congestion.
 19. An apparatus,comprising: a storage and segmentation module configured to receivecomplementary information extracted from an audiovisual stream andincluding digital information suitable for permitting reconstruction ofthe audiovisual stream in conjunction with a modified stream based onthe audiovisual stream after the complementary information is extracted,the storage and segmentation module configured to store thecomplementary information and to segment the complementary informationinto respective segments corresponding to audiovisual elements in unitsof streams of complementary information with a variable size, whereinthe apparatus is configured to transmit the complementary informationinto a distributed network, and wherein the apparatus is furtherconfigured to regulate the quantity of information distributed into adistributed network, taking account of the capacities and restrictionsof the distributed network in terms of storage and of access time. 20.The apparatus according to claim 19, wherein units of streams ofcomplementary information are organized in several layers correspondingto layers of scalability of the audiovisual stream.
 21. The apparatusaccording to claim 19, wherein respective segments correspond to asubjectively coherently entire audiovisual element in units of streamsof complementary information with a variable size.
 22. The apparatusaccording to claim 19, further comprising: an encryption moduleconfigured to encrypt the complementary information prior totransmitting the complementary information into the distributed network.23. An audiovisual distribution system, comprising: at least one localserver, wherein the at least one local server is configured to receivecomplementary information that is extracted from an audiovisual streamand that includes digital information suitable for permittingreconstruction of the audiovisual stream in conjunction with a modifiedstream that is based on the audiovisual stream after the complementaryinformation is extracted, and is configured to provide the complementaryinformation to equipment of a user, wherein the at least one localserver is configured to receive the complementary information in anencrypted form using a first encryption key, is configured to decryptthe complementary information using the first encryption key, and isconfigured to re-encrypt the complementary information using a secondencryption key constructed during an authentication process between theequipment of the user and any local server.
 24. The system according toclaim 23, wherein the at least one local server is configured tore-encrypt the complementary information with the second key adaptivelyas a function of the throughput capacities of the equipment of the user.25. The system according to claim 23, wherein the at least one localserver is configured to generate a session key associated with a sessionestablished with at least one client.
 26. The system according to claim23, wherein the second encryption key is valid solely during the time ofa continuous session between the equipment of the user and at least onelocal server.
 27. The system according to claim 23, wherein at least onelocal server is further configured to generate an access ticket persession, containing information concerning the second encryption key,wherein the second encryption key is valid solely during the session andexpires at the end of the validity of the access ticket.